Method of manufacturing fuel elements



Apnl 15, 1969 R. A. HOWARD ET AL 3,439,073

METHOD OF MANUFACTURING FUEL ELEMENTS Filed Dec. 15, 1966 do w P b -H hp a r G d e d a e r h T Graphite Shell Threaded Graphite Plug ContaipingExpanded Matching Graphite Hemlspheres Wllh Threaded Joint I N V EN TORSRONAL D A HOWA/iQD A T TORNEV United States Patent 3,439,073 METHOD OFMANUFACTURING FUEL ELEMENTS Ronald A. Howard and Arnold A. Kellar,Lawrenceburg, Tenn., and Joseph T. Meers, Cleveland, Ohio, assignors t;llJinion Carbide Corporation, a corporation of New orContinuation-impart of application Ser. No. 388,267, Aug. 7, 1964. Thisapplication Dec. 15, 1966, Ser. No. 602,054

Int. Cl. C21c 21/02 US. Cl. 264-5 8 Claims ABSTRACT OF THE DISCLOSURE Aprocess for making nuclear fuel elements having a fueled core and anunfueled rigid graphite exterior shell which comprises the steps ofpreparing a fueled core mixture containing a nuclear fuel material, ahigh temperature matrix material, expanding graphite and a carbonizablebinder; incorporating said mixture within a rigid graphite shell;heating the resulting composite when a thermosetting binder is employedto cure the binder, and then baking the composite at a temperaturesufiicient to carbonize the binder and expand the expanding graphite.

The use of expanding graphite in the core of the fuel element providesfor continuous contact between the core and outer graphite shell byovercoming the shrinkage which occurs when the fueled core is placed inthe solid graphite exterior shell and baked to carbonize the binder.

This application is a continuation-in-part of application Ser. No.388,267, entitled Fuel Elements, filed Aug. 7, 1964 and now abandoned.

This invention relates to solid nuclear fuel elements and to theconstruction of nuclear fuel elements. More particularly, this inventionrelates to a fuel element having a fueled core comprising a fisisonablematerial disposed in a matrix and having an unfueled carbonaceous cover.

Fission is a process in which the nucleus of a heavy element such asuranium or plutonium, is split into two fragmentary parts by a neutronwhich functions some-what like a projectile. A neutron has no charge andcan easily pentrate into the nucleus of a heavy element. The presence ofthis particle, however, causes the heavy element to become unstable andto split into two fragmentary products. In the process, severaladditional neutrons are released which, in turn, cause other nuclei toundergo fission and this results in a chain reaction. The total mass ofthe nucleus of the fragmentary products is always less than that of theoriginal reactant and the difference in mass is converted to energy ofmotion of the fragmentary products, resulting in heat being produced.This heat can be continuously removed by the passage of a coolantthrough the core of the reactor and may be used as a source of energy.The fission reaction, when viewed on an overall basis, is one of themost efficient sources of energy known today.

Heavy element-containing materials suitable for use as fuels in nuclearfission reactors are well known. For example, there can be utilized assolid nuclear fuels uranium-containing, or plutonium-containingmaterials or substances such as the oxides of uranium e.g., uraniumdioxide (U0 uranic oxide ('UO O etc., and the carbides of uranium e.g.,uranium dicarbide (U0 and uranium monocarbide ('UC); thorium-containingmaterials such as thorium (Th), the carbides of thorium e.g., thoriumdicarbide (TI-1C and thorium monocarbide (Th0) and the oxide of thorium(ThO The solid nuclear fuels may comprise one or more uranium-containingmaterials hoe or one or more thorium containing materials or a mixtureof uranium-containing material and thorium-containing material e.g., amixture of thorium dioxide and an oxide of uranium or a mixture ofthorium dicarbide and uranium dicarbide. The uraniumand/orthorium-containing material may be admixed with a material such aszirconium carbide (ZrC), niobium carbide (NbC) or the like which willimpart chemical and temperature stability to the fuel. Although thoriumis not a fissionable material, when bombarded by neutrons it changesinto uranium becoming fissionable and thus a source of nuclear energy.Therefore, whenever the expression nuclear fuel is used in thespecification and claims it is intended to include non-fissionablematerials such as thorium-containing materials which, when bombarded byneutrons, are transformed into fissionable materials as Well asmaterials which undergo fission under the influence or impact ofneutrons.

The usefulness of many solid nuclear fuels is somewhat curtailed, inmany cases, by the fragmentary products that are produced during thefission proces as heretofore described. These fragmentary productspossess both a high degree of radioactivity and a high degree of kineticenergy. They are capable of penetrating a number of materials resultingin a change of crystal structure, tensile strength, and many otherproperties of the materials. Thus, the presence of these fragmentaryproducts will often restrict the final design and structure of thereactor in the vicinity of the core. Also, these fragmentary byproductsquite often contaminate the coolant as it circulates through the reactorand associated equipment. Consequently, fuel particles are often clad orcoated with aluminum oxide, pyrolytic carbon, or some other materialwhich will reduce the rate of release of fission products during reactoroperation and provide safeguards for handling fuel elements duringfabrication and subsequent handling.

When the fuel elements of a nuclear reactor are to be maintained atelevated operating temperatures, it is impor-tant that the fuel elementsbe mechanically strong and have uniform energy producing and heattransmitting properties. These requirements are important in order thatthe fuel element provide optimum conduction of heat without thedevelopment of isolated areas of elevated temperatures and uniformretention of radioactive materials and fission products.

Heretofore, fuel elements have been made by mixing a nuclear fuel withsome suitable high temperature matrix material, e.g., graphite,beryllium, beryllium oxide, aluminum oxide and the like, and somesuitable binder such as pitch or tar, pressing the resulting admixtureinto the desired shape or form and then baking the formed fuel element.During fabrication of such fuel elements some of the nuclear fuelmaterial becomes located at or near the surface of the fuel element.Since the surface of the fuel element is subject to considerablemechanical abrasion during loading and operation of the reactor, thesurface of the fuel element, and consequently the fuel particles, may beeasily chipped and damaged. Such damage obviously leads to loss of fuelmaterial, and escape of fission products. The ultimate result, ofcourse, is contamination of the reactor coolant and associatedequipment, danger to those handling the fuel elements, and loss ofenergy producing capability.

Damage to the fuel elements and resulting dangers can be overcome atleast in part by providing a protecting cover or cladding for the fuelelement. Nuclear fuel elements are customarily clad or enclosed Within acoating of some suitable metal, ceramic, cermet, carbide, graphite or acombination of these materials. To date, the practice of cladding fuelelements has been found a difficult and costly though necessarypractice. Most commonly employed cladding materials possess one or moredisadvantageous characteristics which limit or circumscribe the generalapplicability of the material. Metal claddings impose an uppertemperature limit on the reactor system, the upper limit being themetallurgical limit of the cladding metal. Cermet and ceramic coatings,while providing for higher operating temperatures, usually fail orrupture as a result of temperature or radiation induced volume and phasechanges. Moreover, such materials usually have low tensile strength, lowthermal conductivity, or brittleness.

One means for providing a fuel element comprising a fueled core and aprotective coating or sheath is 'to provide a hollow sphericalcarbonaceous structure which is filled with a suitable nuclearfuel-matrix-binder admixture. The resulting composite can beconveniently referred to as a fuel element having a fueled core and anunfueled exterior cover.

The term carbonaceous is intended to encompass both carbon and graphite.The term fueled core relates to the mixture of a nuclear fuel materialand a suitable matrix together with a suitable binder, which makes upthe center of a fuel element. The term fuel element relates to thecomposite unit comprising a fueled core and a suitable exterior shell.

The spherical cover member can be conveniently prepared by machining outthe center of a carbonaceous sphere, or by hollowing out the centers ofmatching hemispheres, so as to form concave members which can be filledwith the fuel core material. Alternatively, suitable molding techniquescan be employed to provide concave members directly, without thenecessity of machining out the necessary portions. The thickness of theouter shell will be dictated by the requirements of the reactor system.

The aperture through which the fueled core mixture is introduced into ahollowed out sphere can be easily closed by means of a plug machined toslip or be threaded into the aperture.

The fueled core mixture is prepared by any convenient blending andformulating technique known in the art. Broadly a nuclear fuel materialis blended with a high temperature matrix and a binder in suchproportions as are required to provide convenient handling and formingof the fuel element.

The fueled core mixture is introduced into the protective cover or shellunder pressure sufiicient to provide the desired density. Heat isgenerally supplied to cure any thermosetting binders employed. Then thecomposite fuel element is baked to carbonize the binder and provide thefinal product.

Unfortunately, when a rigid cover is used the baking operation generallyresults in separation of the fueled core from the outer cover. Duringbaking the volatile materials in the fueled core mixture are driven off,the binder is carbonized and there is a resulting decrease in volume.Since the outer cover is a rigid previously baked carbonaceous member,it undergoes no such change in volume. The separation of the fueled corefrom the outer cover is detrimental to the performance of the fuelelement. The gap between the fueled core and the cover reduces thetransfer of heat from the fueled core and greatly reduces thecompressive strength of the fuel element.

It is an object of this invention to provide a fuel element comprising arigid outer cover which surrounds and is intimately bonded to a fueledcore.

It is a further object to provide a mechanically strong fuel elementhaving a fueled core and a rigid outer graphite shell.

It is a still further object to provide a fuel element having a rigidouter shell and a fueled core which has improved thermal conductivitybetween said outer shell and said fueled core.

It is another object to provide a fuel element having a structure whichfacilitates heat transfer from the fueled core to the graphite shell.

It is still another object of the invention to provide a method formaking the herein-described fuel elements, thereby achieving the objectsand advantages described.

These and other related objects are achieved through the use ofexpanding graphite in the construction and fabrication of the fuelelement.

It has been found that certain graphites having a high degree oforientation such as, for example, natural graphites, kish graphite andartificial graphites, for instance, heat treated pyrolytic graphites canbe treated so that the spacing between the superposed layers or laminaecan be appreciably opened up so as to provide a marked expansion in thedirection perpendicular to the layers, that is, in the c direction andthus form an expanded or intumesced graphite structure in which thelaminar character is substantially retained.

In U.S. Patents 1,137,373 and 1,191,383 natural graphite in the form offlake or powder of a size too great to pass through a 200 mesh screen isexpanded by first subjecting the graphite particles for a suitableperiod of time to an oxidizing environment or medium maintained at asuitable temperature. Upon completion of the oxidizing treatment, thesoggy graphite particles or masses are washed with water and then heatedto between about 350 C. and 600 C. to fully expand the graphiteparticles in the c direction. The oxidizing mediums disclosed aremixtures of sulfuric and nitric acids and mixtures of nitric acid andpotassium chlorate.

The term expanding graphite, as used herein, refers to a graphitematerial which has been treated so as to render it capable of expansionupon heating at a temperature above 350 C. To make expanding graphitethe washing step is followed by drying at a temperature below whichexpansion actually takes place. Subsequent heating or baking at propertemperatures results in expansion and the final product is referred toas expanded graphite.

By the above treatment, expansions of the natural graphite particles ofup to about 25 times the original bulk were obtained. There is alsodisclosed that the ex panded natural graphite can be compounded with abinder, e.g., a phenolic resin and the resultant composition compressedor molded into various forms, such as discs, rings, rods, sheets, andthe like.

The use of expanding graphite overcomes the shrinkage of the fueled coreand provides for continuous contact between the fueled core and theouter shell. By eliminating the gap between the fueled core and theshell, heat transfer is enhanced and performance of the fuel element isimproved as well as its mechanical strength and physical integrity.

The expanding graphite can be incorporated directly into the admixturewhich constitutes the fueled core. The amount of expanding graphiteemployed depends on the normal shrinkage of the particular fueled coremixture which is employed. Shrinkage is primarily dependent on thenature of and amount of 'binder in the mixture. In general, satisfactoryresults have been obtained with from 1 to 20 weight percent of expandinggraphite based on the weight of non-fuel components. If too muchexpanding graphite is used, the internal pressure will be excessive andresult in rupturing of the rigid outer shell.

Alternatively, the surface of the cavity of the carbonaceous shell canbe coated with a carbonaceous cement comprising sufficient expandinggraphite to compensate for the shrinkage of the fuel core.

In many instances when long term thermal cycling is anticipated it maybe advantageous to employ both an expanding graphite cement and a fueledcore comprising expanding graphite. Bonding between the outer shell andthe fueled core is thereby greatly enhanced.

Suitable expanding carbonaceous cements comprise finely divided carbonor graphite, a carbonizable resin,

and expanding graphite in proportions which are suitable to provide astrong carbonaceous bond between the fueled core and the rigid graphiteshell after baking. A specific formula for an expanding graphite cementcomprises about 48.4 weight percent each of artificial graphite and aliquid phenolic resin, e.g., a furfural-phenolic resin, and about 3.2weight percent expanding graphite.

In the practice of this invention a mixture comprising a suitable hightemperature matrix material, nuclear fuel particles, expanding graphiteand a carb'onizable binder is molded into a hollow graphite sphere. Heatis then applied to cure the binder. When the binder has been properlycured, the fuel element is baked and the binder carbonized orgraphitized and the expanding graphite is simultaneously expanded. Thefinal fuel element is a unitary composite comprising a fueled core andan outer shell having noseparation between them.

Suitable carbonizable binders include epoxies, phenolics, furfuralalcohol, or coal tar pitches. In general binders having a coking valveof 40% or greater are preferred.

In a preferred embodiment a matrix mixture comprising about 80 weightpercent graphitized petroleum coke, 3 to 8 weight percent expandinggraphite, 12 to 22 weight percent carbonizable binder, all based on thetotal weight of the matrix mixture, is prepared and blended with aspecified amount of nuclear fuel material. The actual amount of nuclearfuel material employed will depend on the specifications to which thefinished fuel element must conform. The resulting blend is then moldedinto a hollow graphite sphere under low molding pressures of less than1000 pounds per square inch. If a thermosetting resin binder is used themolded sphere is then heated to about 100 C. to cure the binder. Theresin can be cured under pressure or if desired the molding pressure canbe released prior to cure. The fuel element is then baked to carbonizethe binder and expand the expanding graphite. During baking thetemperature is raised about 200 C. per hour to a maximum temperature 'ofbetween 1000 C. and 2000 C. The maximum baking temperatures employed arelimited by the decomposition temperature of the nuclear fuel material.

In preparing the fueled core mixture it is essential that the nuclearfuel material be uniformly distributed throughout the mixture. Ingeneral any blending or mixing technique capable of providing uniformdispersion of the nuclear fuel material throughout the mixture issuitable. One method which has been found to provide a uniform mixturecomprises mixing the binder and about one-fifth of the matrix materialto provide a binder rich admixture. The nuclear fuel material is thenblended into this binder rich admixture. Then the balance of the matrixmaterial and the expanding graphite is added and the entire mixture isblended for a period of time sufiicient to provide uniform dispersion ofthe ingredients. In general, it is desirable to add the expandinggraphite as the last ingredient, so that the expanding properties arenot substantially reduced by any physical damage to the expandinggraphite particles caused by abrasion and working of the particles.

FIGURE 1 is a fuel element comprising a hollow spherical graphite shellfilled with a fueled core mixture containing expanded graphite andhaving a threaded aperture sealed with a threaded plug.

FIGURE 2 is a fuel element comprising a hollow spherical graphite shellhaving matching graphite hemispheres fitted together with a threadedjoint and filled with a fueled core mixture containing expanded graphiteand having a threaded aperture sealed with a threaded plug machined tothe curvature of the sphere.

Example I weight percent sized artificial graphite, about 4 weightpercent expanding graphite and about 16 weight percent of a liquidfurfural-phenolic resin. The fueled core contains the above-describedformulation and about 25 grams of pyrolytic carb'on coated particles ofuranium carbide in a uniform blend. The blend is then molded into a 4.4centimeter diameter cavity of a graphite sphere which has a 6 centimeterdiameter.

Example II A fueled core mixture was prepared by blending theconstituent materials in the following manner.

A carbonizable liquid resin was added cold into a suitable mixer andone-fifth of the artificial graphite added thereto and blended toprovide a resin rich mixture. Then a sufiicient amount of pyrolyticcarbon coated uranium carbide particles to provide about 25 grams ofsuch particles for each sphere was added and blended with the resin richmixture. Then the balance of the artificial graphite was added andblended. The expanding graphite was added last and the total mixtureblended for about one hour.

Example 111 A fuel element comprising a fueled core and an unfueledouter shell was prepared in the following manner. A mixture comprising45.10 grams of sized purified artificial graphite, 1.81 grams ofpurified expanding graphite, 9.02 grams of furfural-phenolic liquidresin, and 24.20 grams of pyrolytic carbon coated particles of uraniumdicarbide was prepared by blending one-fifth of the artificial graphitewith the liquid resin in a cold mixer. The pyrolytic carbon coated fuelparticles and the balance of the artificial graphite were added to therunning mixer. Then the purified expanding graphite was introduced intothe mixer while still running. The resulting admixture was blended forabout 30 minutes. An 80.1 gram portion of the blended admixture was thenweighed out and forced into a hollow graphite sphere through a threadedaperture under a pressure of about 300 pounds per square inch. Thethreaded aperture was then sealed with a threaded plug coated with anexpanding graphite cement, comprising 48.4 weight percent artificialgraphite, 48.4 weight percent furfu ral-phenolic liquid resin and 3.2weight percent expanding graphite. The sphere was then cured at C. for 8hours, after which the top of the plug was machined to the curvature ofthe sphere. The sphere was then baked to 1450 C. by raising thetemperature from 50 C. to 500 C. at a rate of degrees per hour, and from500 C. to 1450 C. by raising the temperature 250 degrees per hour. Thetemperature was held at 1450 C. for 30 minutes.

What is claimed is:

1. The method of making a fuel element for use in a nuclear reactorwhich comprises;

(a) making a mixture for the fuel core which includes a nuclear fuelmaterial, a suitable high temperature matrix material and expandinggraphite and a carbonizable binder;

(b) molding said mixture within a rigid graphite exterior shell, andheating the molded unit when a thermosetting binder is employed to curethe binder;

(c) baking the molded unit to a temperature sufiicient to carbonize thebinder and expand the expanding graphite.

2. The method as in claim 1 wherein the binder is a thermosettingbinder.

3. The method of making a fuel element for use in a nuclear reactorwhich comprises:

(a) making a mixture comprising suflicient quantity of a hightemperature matrix material, from about 1 to about 20 weight percentexpanding graphite, and from about 12 to about 22 weight percent of acarbonizable binder;

(b) blending with said mixture a selected amount of a nuclear fuelmaterial;

(c) molding the resulting blended admixture within a graphite shell at apressure between 200 and 300 pounds per square inch;

(d) heating the resulting molded unit when a thermosetting binder isemployed at a temperature sufilcient to cure the binder;

(e) and then baking the molded unit at a temperature between 1000 C. and2000" C. to carbonize the binder and expand the expanding graphite.

4. The method as in claim 3 wherein the binder is a thermosettingbinder.

5. In a process for making nuclear fuel elements having a fueled coreand an unfueled graphite shell which comprises the steps of preparing afueled core mixture containing a nuclear fuel material, a hightemperature matrix material and a carbonizable binder; incorporatingsaid mixture within a graphite shell; heating the resulting compositewhen a thermosetting binder is employed to cure the binder, and thenbaking the composite at a temperature sufficient to carbonize thebinder; the improvement which comprises employing expanding graphite asa constituent of the fueled core mixture in amounts sulficient toprevent separation of the fueled core from the graphite shell duringbaking.

6. The method as in claim 5 wherein the binder is a thermosettingbinder.

7. In a process for making nuclear fuel elements having a fueled coreand an unfueled graphite exterior shell which comprises the steps ofpreparing a fueled core mixture containing a nuclear material dispersedthroughout a high temperature matrix and a carbonizable binder;incorporating said mixture within a graphite exterior shell; heating theresulting composite when a thermosetting binder is employed to cure thebinder, and then baking the composite at a temperature sufiicient tocarbonize the binder; the improvement which comprises employingexpanding graphite as a constituent of the fueled core mixture inamounts suificient to compensate for shrinkage of the fueled core uponcarbonization of the binder.

8. The method as in claim 7 wherein the binder is a thermosettingbinder.

References Cited UNITED STATES PATENTS 3,212,989 10/1965 Fitzer et a1.17671 3,260,651 7/1966 Gress et al. 176-90 3,284,314 11/1966 Rachor etal. l7690 CARL D. QUARFORTH, Primary Examiner.

MELVIN J. SCOLNICK, Assistant Examiner.

U.S. Cl. X.R.

